During the fabrication of integrated circuits, plasma etching is widely used to etch through a dielectric layer in order to form a hole through which an underlying silicon surface can be conatcted. Plasma etching is normally effected by means of a high frequency (RF) discharge of a gas containing atomic species that attack the dielectric--typically one of the halogens.
Plasma etching is thus a combination of chemical effects by molecules whose activity has been increased by virtue of their being ionized and/or transformed into free radicals or atoms and physical effects resulting from ion bombardment. Both mechanisms are effective in removing material but the bombardment effects often leave behind a residue of surface damage.
In FIG. 1 we illustrate schematically a silicon substrate 1 on whose surface is dielectric layer 2 in which a hole needs to be etched so that the silicon can be contacted. The area that is to be etched is defined by opening 4 in photoresist layer 3. The latter was deposited onto 2 and then exposed through a suitable optical mask so that, after development, resist 3 covered the dielectric everywhere except at 4.
FIG. 2 illustrates the appearance of the structure after the completion of plasma etching. Typically this would have been at a high RF power level, between about 1,200 and 1,400 watts, using a gas such as trifluoromethane in argon at a pressure of about 0.3 torr and a chamber wall temperature of about 40.degree. C. As shown, hole 21 has been etched through dielectric layer 2 but there is significant surface damage 24 to the exposed silicon surface.
In order to minimize the extent of surface damage (such as 24) it has been the practice in the prior art to sometimes follow the plasma etch described above with a so-called soft etch wherein the RF power level is reduced to a significantly lower level (typically about 160 watts in this example) and maintained at this reduced level for about 10 seconds. Following the soft etch, the structure was exposed to a plasma of pure oxygen for about 1 minute. This has the effect of removing most of the photoresist. Any photoresist that still remained was then removed using a wet etch such as a mixture of sulphuric acid and hydrogen peroxide in water.
The problem with the prior art procedure that we have described above is that the soft etch step does not remove all the surface damage. Even though it is performed at reduced power relative to the main etch there are still present energetic ions that can damage the silicon surface. Additionally, removal of the photoresist in an oxygen plasma, while effective for that purpose, also introduces some surface damage since, once again, energetic ions are present. The net result of these practices is illustrated in FIG. 3 which shows that although surface damage 31 has been reduced relative to prior damage 24 (FIG. 2) it has still not been eliminated. The presence of such residual surface damage is found to correlate with an increase in the contact resistance between the silicon and a conductor deposited in the hole.
A secondary problem, associated with the prior art procedures described above, is that the match box (used to match the impedance of the power supply to that of the discharge) is optimized for high power operation and a sudden switch to low power can cause damage to the match box.
The present invention provides a way to remove the last remnants of surface damage to the silicon while also removing photoresist. In an examination of the published prior art nothing was found that anticipated the present invention. Several references were, however, found to be of interest. For example, ICP (induced Coupled Plasma) etching is described by Chang and Sze in ULSI Technology pp. 348-351 (McGraw Hill 1997) while the removal of photoresist by plasma etching is discussed by Elliott in Integrated Circuit Fabrication Technology pp. 305-307 (McGraw Hill 1982).
Chen et al. (U.S. Pat. No. 5,368,710 Nov. 1994) describe generation of a plasma by external means and transmitted through a dielectric window whose thickness varies so that the plasma is weakest where the window is thickest. Shortes et al. (U.S. Pat. No. 4,341,592 Jul. 1982) use an atmosphere that includes ozone for the removal of photoresist. Ohmi (U.S. Pat. No. 5,272,417 Dec. 1993) use two separate power sources that operate at different frequencies and drive separate (but opposing) electrodes, to generate a plasma.